Charge pump system

ABSTRACT

A system includes a charge pump system having a plurality of enable signal input terminals and an output terminal, the charge pump system configured to provide an output voltage at the output terminal; and a detection circuit connected to the enable terminals and the output terminal of the charge pump system, the detection circuit configured to compare the charge pump system output voltage to a plurality of predefined input detection voltage levels, and to selectively output a plurality of enable signals to the charge pump system enable signal input terminals in response to the comparison.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 16/657,221,filed Oct. 18, 2019, the disclosure of which is hereby incorporated byreference in its entirety.

BACKGROUND

Charge pump circuits are generally used for generating higher voltagesfrom low voltage inputs. It is typically used for providing a steadylevel of output voltage. The provision of these higher voltages iscritical in many low voltage applications such as providing the biasvoltage for a memory cells such as a RRAM cell.

A conventional charge pump system used to provide bias voltages for amemory cell may comprise a charge pump to generate the higher voltagefollowed by a filter, typically a filter, and a low dropout regulatorcircuit to filter out the noise and provide a stable, low ripple voltageto the memory cell.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a block diagram illustrating an example of a charge pumpsystem in accordance with some embodiments.

FIG. 2 is a block diagram illustrating an example of a resistive randomaccess memory (RRAM) circuit in accordance with some embodiments.

FIG. 3 is a circuit diagram illustrating an example of the detectioncircuit of FIG. 1 in accordance with some embodiments.

FIG. 4 is a circuit diagram illustrating an example of the charge pumpof FIG. 1 in accordance with some embodiments.

FIG. 5 a circuit diagram illustrating another example of the charge pumpof FIG. 1 in accordance with some embodiments.

FIG. 6 is a circuit diagram illustrating an example of a tunable ringoscillator in accordance with some embodiments.

FIG. 7 is a flow diagram illustrating an example of a method inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In some memory cells, such as RRAM cells, the word line bias voltage mayneed to be boosted to a higher level than the voltage provided by thepower supply voltage. Typically, a charge pump circuit is used to supplythe higher voltage as necessitated by the memory cell to function withinthe proper operating range. In such cases, the charge pump circuit isdesigned to operate under the highest possible current load conditions.However, when the current load is low, the output voltage of such chargepumps may include higher ripple amplitudes than desired. Therefore,under such conditions, circuits to reduce ripple amplitude of the chargepump output voltage may be employed.

Typically, memory cells, during write operations, may need a biasingvoltage that may be larger than the supply voltage to operate within theproper range. In such cases, a charge pump is used to boost the voltagelevel higher. However the output voltage generated by the charge pumpmay not stable enough for biasing memory cells operations. Therefore, anRC filter and an LDO are sometimes used to smooth out the output of thecharge pump before being used to bias the one or more memory cell writevoltages. The RC Filter is designed to filter out the ripples from theoutput signal of the charge pump. However, an RC filter may occupysubstantial area, induce extra IR drop and increase the current load.

FIG. 1 illustrates an example system 100 in accordance with disclosedembodiments that provides a low ripple voltage signal. Voltage ripple isthe residual periodic variation of DC voltage when it is derived from anAC power supply. Voltage ripple is measured as peak-to-peak voltageamplitudes. In some examples, the system 100 can reduce the outputripple voltage of the charge pump system 120 such that the peak to peakripple voltage measures +/−20 mV. In other examples, the output ripplevoltage may measure +/−10 mV or +/−5 mV. The system 100 includes adetection circuit 110 and a charge pump system 120. The charge pumpsystem 120 receives a plurality of enable signals 112, 114, 116, 118 asinputs to control the operation of the charge pump system. In oneexample, the plurality of enable signals includes four enable signals,112, 114, 116, 118. However, any number of enable signals may begenerated by the detection circuit 110. The number of enable signals maybe based on the voltage boost capacity of the one or more charge pumpsused within the charge pump system. Examples of disclosed charge pumpsystems are described below in relation to FIG. 4 and FIG. 5.

The charge pump system 120 is configured to generate a boosted level ofoutput voltage at a node 130. The charge pump output voltage at node 130of the charge pump system 120 can then be fed into a load 140. In theillustrated example, the load 140 is a circuit that uses the boosted lowripple voltage for its operation, such as a memory circuit 210, whichtypically uses a stable, low ripple voltage signal for its write biasvoltage during write operations. The charge pump output voltage at node130 is also used as an input signal for the detection circuit 110 aspart of a feedback loop. FIG. 4 and FIG. 5 show two differentembodiments of the charge pump system connected to the detection circuit110 that can be used to generate the low ripple output signal as used bya load 140.

In some examples, the detection circuit 110 is configured to monitor thecharge pump output voltage at node 130. The detection circuit 110includes a plurality of input detection voltage levels 102, 104, 106,108, and the charge pump output voltage from node 130 as inputs. Thedetection circuit 110 outputs a plurality of enable signals, 112, 114,116, 118. Although four input detection voltage levels 102, 104, 106,108 and four enable signals 112, 114, 116, 118 are shown in the exampleillustration of the system 100, it is understood that the number ofinput detection voltage levels and enable signals are variable and othernumbers of input detection voltage levels and enable signals are withinthe scope of the present disclosure. The plurality of enable signals,112-118 are then included as inputs to the charge pump system 120. Theplurality of enable signals are used to control the operation of the oneor more charge pumps that are part of the charge pump system 120. Thedetection circuit 110 is described in greater detail in relation to FIG.3.

FIG. 2 illustrates an embodiment of the load 140 that includes thememory circuit 210. Memory devices are used to store information insemiconductor devices and systems. The illustrated memory circuit 210includes a plurality of Resistive Random Access Memory (RRAM) cells 212arranged in an array of rows and columns. RRAM memory cells arenon-volatile memory cells that store information based on changes inelectric resistance. In general, each RRAM cell 212 includes a storagenode in which a bottom electrode, a resistive switching layer and a topelectrode may be sequentially stacked. The resistance of the resistiveswitching layer varies according to an applied voltage. An RRAM cell canbe in a plurality of states in which the electric resistances aredifferent. Each different state may represent a digital information. Thestate can be changed by applying a predetermined voltage or currentbetween the electrodes. A state is maintained as long as a predeterminedoperation is not performed.

For discussion purposes, FIG. 2 shows four RRAM memory cells 212.However, a typical RRAM memory array would include many more RRAM cells.The RRAM cells 212 are arranged within the RRAM array in rows and/orcolumns. RRAM cells 212 within a row of the RRAM array are operablycoupled to a word line WL1 or WL2, respectively, while RRAM cells 212within a column of the RRAM array are operably coupled to a bit line BL1or BL2 and a common source line CSL. The plurality of RRAM cells arerespectively associated with an address defined by an intersection of aword line WL1 or WL2 and a bit line BL1 or BL2.

Each of the RRAM cells 212 includes an RRAM resistive element 214 and anaccess transistor 216. The RRAM resistive element 214 has a resistivestate that is switchable between a low resistive state and a highresistive state. The resistive states are indicative of a data value(e.g., a “1” or “0”) stored within the RRAM resistive element 214. TheRRAM resistive element 214 has a first terminal coupled to one of thebit lines BL1 or BL2 and a second terminal coupled to the accesstransistor 216. The access transistor has a gate coupled to one of theword lines WL1 or WL2, a source coupled to the common source line CSLand a drain coupled to the second terminal of the RRAM resistive element214. By activating the word line WL1 or WL2, the access transistor 214is turned on, allowing for the common source line CSL to be coupled tothe second terminal of the RRAM resistive element 214.

The RRAM array is configured to read data from and/or write data to theplurality of RRAM cells 212. A word line signal (e.g., a current and/orvoltage) is applied to one of the word lines WL1-WL2 based upon a firstaddress ADDR1 received by a word line decoder, a bit line signal isapplied to one of the plurality of bit lines BL1-BL2 based upon a secondaddress ADDR2 by a bit line decode. In some examples, and a commonsource line signal is applied to the common source line CSL based on thesecond address ADDR2, and in other examples the CSL signal is applied tothe common source line CSL based upon a third address ADDR3.

By selectively applying signals to the word lines WL1-WL2, the bit linesBL1-BL2, and the common source line CSL, forming, set, reset, and readoperations may be performed on selected ones of the plurality of RRAMcells 212. For example, to read data from a particular RRAM cell 212, aword line signal (e.g., voltage) is applied to the word line WL1, a bitline signal (e.g., voltage) is applied to the bit line BL1, and a sourceline signal (e.g., voltage) is applied to the common source line CSL.The applied signals cause a read sense amplifier to receive a signal(e.g., voltage) having a value that is dependent upon a data state ofthe RRAM cell 212. The sense amplifier is configured to sense thissignal and to determine the data state of the selected RRAM cell basedon the signal (e.g., by comparing a received voltage to a referencevoltage).

FIG. 3 illustrates an embodiment of the detection circuit 110. In oneexample, the detection circuit is implemented with a voltage comparatorthat is configured to include a plurality of input voltage leveldetection circuit branches. The detection circuit 110 includes a currentbias circuit 305 that supplies a shared current to the plurality ofinput voltage level detection circuit branches 310, 312, 314, 316. Whilean embodiment including four input voltage level detection circuitbranches 310, 312, 314, 316 is illustrated in FIG. 3, it is understoodthat the detection circuit 110 may be designed to include more or fewerinput voltage level detection circuit branches. Each input voltage leveldetection circuit branch is associated with an input detection voltagelevel and an enable output signal, for example input detection voltagelevel 102 and enable output signal 112 in the case of the input voltagelevel detection circuit branch 310 shown in FIG. 3.

The current bias circuit 305 includes a current source 320 that isconnected to the drain terminal of an NMOS transistor 322. The source ofthe NMOS transistor is connected to ground. The gate terminal of theNMOS transistor 322 is tied to the drain terminal of the NMOS transistor322, which is connected to the current source 320. The gate terminal ofthe NMOS transistor 322 is also tied to the gate terminal of anotherNMOS transistor 360. The current source 320, the NMOS transistor 322 andthe NMOS transistor 360 are arranged in a current mirror configuration.

In the illustrated example, the input detection voltage levels 102, 104,106, 108 are pre-determined voltage values that are dependent on thevoltage requirements of the load 140 connected to the output of thecharge pump system and the voltage boost capabilities of the one or morechare pumps included in the charge pump system 120. In one example, inorder to maintain a low ripple charge pump output voltage at 2.5V, thefour input detection voltage levels 102, 104, 106, 108 are set at 2.68V,2.62V, 2.56V and 2.5V. The input detection voltage levels arepre-determined to ensure that the charge pump output voltage at node 130stays adequately higher than the current load requirements of the load140.

Each of the input voltage level detection circuit branches 310, 312, 313and 316 comprises a PMOS transistor and an NMOS transistor. For example,the input voltage level detection circuit branch 310 comprises PMOStransistor 330 and NMOS transistor 332. The source terminal of the PMOStransistors 330, 332, 334, 336 of the respective input voltage leveldetection circuit branches 310, 312, 314, 316 are each connected to arail voltage terminal 350 configured to receive a supply voltage VDIO.The drain terminal of each PMOS transistor 330, 332, 334, 336 is tied tothe drain terminal of its respective NMOS transistor 340, 342, 344, 346and is also connected to respective enable signals 112, 114, 116, 118.The source terminals of each of the NMOS transistors 340, 342, 344, 346are connected to the drain terminal of the NMOS transistor 360. Thesource terminal of the NMOS transistor 360 is connected to ground.

The gate terminals of the PMOS transistors 330, 332, 334, 336 are tiedtogether and are also connected to the gate terminal and drain terminalof a PMOS transistor 338 that is part of a charge pump output voltagelevel detection circuit 318. The gate terminals of the NMOS transistors340, 342, 344, 346 are configured to receive the respective inputdetection voltage levels 102, 104, 106, 108. The gate terminal of theNMOS 348 of the charge pump voltage level detection circuit 318 isconnected to the charge pump output voltage at node 130. The drainterminal of the NMOS transistor 348 is tied to the drain terminal of thePMOS transistor 338 and the source terminal of the NMOS transistor istied to the source terminal of the NMOS transistors 340, 342, 346, 348and connected to the drain terminal of the NMOS transistor 360.

The input voltage level detection circuit branches 310, 312, 314, 316are in a current mirror configuration with the charge pump outputvoltage level detection circuit 318. The current mirror configurationprovides a trans-impedance element such that the input voltage leveldetection circuit branches 310-316 have a voltage output 112-118.

In one example, the PMOS transistors 330, 332, 334, 336 of each of theplurality of input voltage level detection circuit branches 310, 312,314, 316 are designed to have the same size as the PMOS transistor 338,and the NMOS transistors 340, 342, 344, 346 of each of the plurality ofinput voltage level detection circuit branches 310, 312, 314, 316 aredesigned to have the same size as the NMOS transistor 348. The size ofsuch MOS devices is defined, for example, by the width and lengthparameters of the MOS devices. For instance, a wider device allows morecurrent flow. The width and length parameters may be selected such thatthe MOS devices have sufficient drive strength to supply the desiredcurrent and leakage levels. In addition to the matching sizes of thetransistors, the layout of the PMOS transistors are all configured tomatch and the layout of the NMOS transistors are all configured tomatch.

In the illustrated example, the detection circuit 110 is implementedusing a voltage comparator circuit 300. The charge pump output voltageat node 130 is tied to the gate terminal of the NMOS device 348 of thecharge pump voltage level detection circuit 318 as part of a feedbackmechanism. The NMOS device's 348 operation is thus controlled by chargepump output voltage at node 130. The NMOS transistor 360 is configuredto be in a current mirror configuration with the NMOS transistor 322.Accordingly, the current flowing through NMOS transistor 322 matches thecurrent flowing through NMOS transistor 360. Thus the current flowingthough each branch 310-318 totals to match the current source 320.

The detection circuit 110 is configured to compare the voltage at node130 to each of the plurality of input detection voltage levels 102, 104,106, 108. For example, if the charge pump operation cannot supply thenecessary current load, the charge pump output voltage at node 130 willdescend. Upon comparing the charge pump output voltage at node 130 withthe first detection voltage level 102, if the voltage level 102 is lowerthan the charge pump output voltage at node 130, the correspondingenable signal 112 goes high. The same process happens with the otherinput voltage level detection circuit branches 312, 314, and 316 aswell. As the load 140 draws a higher current load, more enable signals112-118 are pulled high to subsequently turn on more charge pumpcircuits.

FIG. 4 illustrates an example low ripple voltage signal system 400 inaccordance with one embodiment of the charge pump system 120. The lowripple voltage signal system 400 comprises the detection circuit 110,the charge pump system 120 and the load 140. The illustrated embodimentof the charge pump system 120 includes a plurality of charge pumps 410,420, 430, 440. Although the embodiment of the charge pump system 120from FIG. 4 shows four charge pumps 410, 420, 430, 440, it is understoodthat the charge pump system 120 may include more or fewer charge pumps.Each charge pump receives a respective one of the enable signals 112,114, 116, 118, and a fixed frequency clock signal 450 as inputs. Theoutputs of the plurality of charge pumps are tied together and serve asthe input to the load 140.

The charge pumps 410, 420, 430, 440 operate based on the state of theenable signals 112, 114, 116, 118 from the detection circuit 110. Forexample, if the enable signal 112 received by the charge pump 410 ishigh, the charge pump 410 will generate a boosted level of charge pumpsystem output voltage at node 130. The voltage level at node 130 can beadjusted by turning one or more of the charge pumps 410, 420, 430, 440on or off. The adjustment to correct any fluctuations or ripples in thecharge pump system output voltage is done automatically using thedetection circuit 110 coupled with the feedback from the charge pumpsystem output voltage at node 130. The detection circuit 110 monitorsthe charge pump system voltage level at the node 130 and controls thecharge pumps 410, 420, 430, 440 accordingly. If the charge pump systemoutput voltage level at node 130 drops, the multiple voltage detectioncircuits sense the drop and triggers the relevant input voltage leveldetection circuit branches to turn the corresponding enable signal high.The enable signals each control a charge pump that subsequently turns onto output a boosted voltage. The number of charge pumps that are turnedon and the operating capacity of each charge pump is directlyproportional to the amount of voltage boost received by the charge pumpsystem output voltage level at node 130. For example, if each chargepump has the operating capacity to produce a voltage boost of 0.06V, andthe charge pump system output voltage drops by 0.18V, then three of thecharge pumps will be turned on automatically to boost the charge pumpsystem output voltage by 0.18V. Thus, any drops in the charge pumpsystem output voltage is corrected by the boost in voltage provided byturning on the appropriate charge pumps within the charge pump system120.

For example, looking at the overall charge pump system 120, when acharge pump is turned on, the charge pump output voltage at node 130gets a boost in voltage. When the charge pump output voltage at node 130hits the second detection voltage level 104, the enable signal 114 goeshigh and enables a second charge pump to turn on, which in turn booststhe charge pump output voltage at node 130 even more. If the currentload is larger than the operating capacity of the charge pump, theoutput voltage will keep descending and trigger more charge pumps untilthe charge pump system operates to surpass the current load. Sinceoperation of the charge pump system is always adequately higher than thecurrent load, this can effectively suppress over shoots of the chargepump.

FIG. 5 illustrates another example low ripple voltage signal system 500in accordance with another embodiment of the charge pump system 120. Thelow ripple voltage signal system 500 comprises the detection circuit110, an alternative embodiment of the charge pump system 120 and theload 140. The embodiment of the charge pump system 120 shown in FIG. 5includes a variable frequency generator 510 and a charge pump 520. Oneexample of the variable frequency generator 510 may be implemented usinga tunable ring oscillator. An example implementation of the tunable ringoscillator is described in relation to FIG. 6, which is discussedfurther below. The charge pump 520 is configured to generate a boostedlevel of output voltage at the node 130. This output signal at the node130 is also used as an input signal for the detection circuit 110 aspart of a feedback loop as discussed above. The detection circuit 110shown in FIG. 5 is arranged to monitor the output voltage level of thecharge pump 520 at the node 130. The output of the detection circuit 110is a set of enable signals 112, 114, 116, 118 for the input of thevariable frequency generator 510. The frequency of the variablefrequency generator 510 can be adjusted according to the plurality ofenable signals 112, 114, 116, 118 that are provided by the detectioncircuit 110. This also means that the current load provided as the inputof the charge pump 520 can be adjusted according to the output voltagelevel of the charge pump 520 at the node 130, thus stabilizing theoutput voltage level of the charge pump 520.

For example, as the output voltage of the charge pump 520 at node 130drops, the detection circuits senses the drop and triggers the relevantinput voltage level detection circuit branches to turn the correspondingenable signal high. The number of enabled level detection circuits istranslated to the digital signal used to control the frequency selectionof the variable frequency generator's 510 output clock signal. In oneexample, the variable frequency generator may comprise a tunable ringoscillator circuit. A lower charge pump output voltage at node 130 maystart a faster ring oscillator to speed up the charge pumping operationand induce a larger output current to increase the dropping voltagelevel, thus regulating the voltage drop at the charge pump output.

FIG. 6 illustrates an example of a tunable ring oscillator 600 that maybe implemented within the charge pump system 120 shown in FIG. 1. Thetunable ring oscillator 600 is a loop that comprises a plurality of NANDgates 610, 612 and inverters 620 a-620 g (collectively inverters 620).The oscillator 600 is configured to provide a frequency output to thecharge pump 520 at a certain frequency. The ring oscillator 600 furtherincludes a plurality of input NAND gates 602, 604, 606.

In this example, three different output frequencies may be provided.However, it is understood that more or less number of frequencies can bemade available by connecting more number NAND gates in addition to NANDgates 602, 604 and 606 and routing more enable signals to the NAND gateinputs. NAND gates 602, 604, and 606 are implemented to control thefrequency selection of the tunable ring oscillator 600 based on theenable signals 112, 114, 116 from the detection circuit 110. Each of theNAND gates 602, 604, 606 has one input connected to receive acorresponding one of the enable signals 112, 114, 116 from the detectioncircuit 110 shown in FIG. 3. The NAND gates 602, 604, 606 further have asecond input connected to receive the oscillator output 608 as afeedback signal. For the NAND gates 602 and 604, and the outputs aretied respectively to an input of the NAND gates 610 and 612. The NANDgates 610 and 612 are used to determine how many sets of inverters 620are connected to the loop.

In the tunable ring oscillator 600, the frequency can be adjustedaccording to the one or more enable signals 112, 114, 116 from thedetection circuit 110 shown in FIG. 3. The inverters 620 function asdelay elements, and the tunable ring oscillator 600 operates by usingthe time delays implemented by the inverters 620 to change the frequencyof the output signal 608. If a higher number of inverters 620 areconnected, the oscillator 600 provides a lower frequency output signalat node 608, while if a lower number of inverters 620 are connected, theoscillator 600 provides a higher frequency output signal at node 608.With different combinations of enable signals controlling the NAND gates602, 604, 606, 610, 612 the number of inverters 620 that are part of thering can be adjusted and thus the frequency of the tunable ringoscillator 600 can be adjusted. If a higher number of enable signals areturned on, then the oscillator output signal has a lower frequency. Iffewer enable signals are turned on, then the oscillator output signalhas a higher frequency.

FIG. 7 illustrates a method 700 in accordance with disclosedembodiments. At step 710, an output voltage at an output node 130 of acharge pump 120 is compared to a plurality of predefined input detectionvoltage levels at input terminals 102-104. At step 712, one or more of aplurality of enable signals at enable terminals 112-118 are turned onbased on the comparison of step 710. Based on the enable signals thatare turned on, the charge pump 120 is controlled at step 714 to modifythe output voltage of the charge pump as shown in step 716. For example,a clock signal frequency may be selected from a plurality ofpredetermined frequencies by the variable frequency generator 510 shownin FIG. 5, and the selected clock frequency is output to the charge pump120. In other examples, one or more of a plurality of charge pumps 410may be activated based on the enable signals as shown in FIG. 4.

Thus, disclosed embodiments provide a charge pump system that minimizesthe ripple amplitude of a charge pump circuit output voltage, withoutincreasing the circuit area or the current load. Some disclosedembodiments include a detection circuit and feedback mechanism to sensea drop in voltage automatically enable a charge pump system tocompensate for the drop in voltage, thus reducing ripple amplitude inthe voltage signal.

In accordance with some embodiments, a system that reduces ripple of acharge pump system output voltage is disclosed. The system comprisescharge pump system having a plurality of enable signal input terminalsand an output terminal, the charge pump system configured to provide anoutput voltage at the output terminal; and a detection circuit connectedto the enable terminals and the output terminal of the charge pumpsystem, the detection circuit configured to compare the charge pumpsystem output voltage to a plurality of predefined input detectionvoltage levels, and to selectively output a plurality of enable signalsto the charge pump system enable signal input terminals in response tothe comparison.

In accordance with further embodiments, a memory system that includes alow ripple input voltage is disclosed. The memory cell system comprises:an array of memory cells; a plurality of bit lines connected to thearray of memory cells; a plurality of charge pumps, each having anenable signal input terminal and an output terminal coupled to theplurality of bit lines; and a detection circuit connected to the enableterminals and the output terminal of the charge pump, the detectioncircuit configured to compare the charge pump output voltage to aplurality of predefined reference voltage levels, and to selectivelyoutput a plurality of respective enable signals to the charge pumpenable signal input terminals in response to the comparison.

In accordance with other embodiments, a method for generating a lowripple charge pump system output voltage is disclosed. The methodcomprises: comparing an output voltage of a charge pump to a pluralityof predefined input detection voltage levels; based on the comparison,causing one or more of a plurality of enable signals to turn on; basedon the enable signals that are turned on, selecting a clock signalfrequency from a plurality of predetermined frequencies; outputting theselected clock frequency to the charge pump; and modifying the outputvoltage of the charge pump based on the selected clock frequency.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A system comprising: a variable frequencygenerator having a plurality of enable signal input terminals thatreceive a plurality of enable signals and an output terminal; a chargepump device with a charge pump input terminal connected to the outputterminal of the variable frequency generator and a charge pump outputterminal, wherein the charge pump device is configured to receive theselected clock frequency from the variable frequency generator at thecharge pump input terminal and generate a charge pump output voltage atthe charge pump output terminal based on the selected clock frequency;and a detection circuit connected to the enable signal input terminalsof the variable frequency generator and the charge pump output terminal,the detection circuit configured to compare the charge pump outputvoltage to a plurality of predefined input detection voltage levels, andto selectively output the plurality of enable signals to the enablesignal input terminals of the variable frequency generator in responseto the comparison.
 2. The system of claim 1, wherein the detectioncircuit comprises a voltage comparator circuit including a plurality ofinput level detection circuit branches, each branch configured tocompare the charge pump output voltage to a corresponding one of theplurality of predefined input detection voltage levels, and wherein eachinput level detection circuit branch is connected to a respective one ofthe enable signal input terminals.
 3. The system of claim 2, wherein thedetection circuit includes a bias current generator circuit configuredto provide a bias current to each of the plurality of input leveldetection circuit branches.
 4. The system of claim 2 wherein each inputlevel detection circuit branch comprises a first PMOS transistor and afirst NMOS transistor, wherein: the source terminal of the first PMOStransistor is connected to a rail input voltage; the gate terminal ofthe first PMOS transistor is connected to the drain of a second NMOStransistor whose gate is connected to the charge pump output terminal;the drain terminal of the first PMOS transistor is connected to one ofthe plurality of enable signal input terminals and to the drain terminalof the first NMOS transistor; and the gate terminal of the first NMOStransistor is connected to one of the plurality of predefined inputdetection voltage levels.
 5. The system of claim 4, wherein a sourceterminal of the first NMOS transistor is configured to connect to adrain terminal of a current mirror NMOS transistor that is part of acurrent mirror configuration.
 6. The system of claim 4, wherein the PMOStransistors of the each of the plurality of input level detectioncircuit branches have the same size, and wherein the NMOS transistors ofthe each of the plurality of input level detection circuit branches havethe same size.
 7. The system of claim 1, wherein the variable frequencygenerator includes a tunable ring oscillator circuit.
 8. The system ofclaim 7, wherein the tunable ring oscillator circuit comprises aplurality of NAND gates and inverters connected in a loop.
 9. The systemof claim 1, wherein the variable frequency generator is configured tooutput a selected clock frequency based on a configuration of the enablesignals;
 10. The system of claim 1, further comprising: an array ofmemory cells; a plurality of bit lines connected to the memory cells;wherein the output terminal of the charge pump system is coupled to theplurality of bit lines.
 11. The system of claim 10, wherein the memorycells include resistive random access memory (RRAM) cells.
 12. A memorysystem comprising: an array of memory cells; a plurality of bit linesconnected to the array of memory cells; a variable frequency generatorhaving a plurality of enable signal input terminals that receive aplurality of enable signals and an output terminal; a charge pump devicewith a charge pump input terminal connected to the output terminal ofthe variable frequency generator and a charge pump output terminal,wherein the charge pump output terminal is coupled to the plurality ofbit lines and the charge pump device is configured to receive theselected clock frequency from the variable frequency generator at thecharge pump input terminal and generate a charge pump output voltage atthe charge pump output terminal based on the selected clock frequency;and a detection circuit connected to the enable signal input terminalsof the variable frequency generator and the charge pump output terminal,the detection circuit configured to compare the charge pump outputvoltage to a plurality of predefined input detection voltage levels, andto selectively output the plurality of enable signals to the enablesignal input terminals of the variable frequency generator in responseto the comparison.
 13. The memory system of claim 12, wherein thedetection circuit comprises a voltage comparator circuit including aplurality of input level detection circuit branches, each branchconfigured to compare the charge pump output voltage to a correspondingone of the plurality of predefined input detection voltage levels, andwherein each input level detection circuit branch is connected to arespective one of the enable signal input terminals.
 14. The memorysystem of claim 12 wherein each input level detection circuit branchcomprises a first PMOS transistor and a first NMOS transistor, wherein:the source terminal of the first PMOS transistor is connected to a railinput voltage; the gate terminal of the first PMOS transistor isconnected to the drain of a second NMOS transistor whose gate isconnected to the charge pump output terminal; the drain terminal of thefirst PMOS transistor is connected to one of the plurality of enablesignal input terminals and to the drain terminal of the first NMOStransistor; and the gate terminal of the first NMOS transistor isconnected to one of the plurality of predefined input detection voltagelevels.
 15. The memory system of claim 14, wherein a source terminal ofthe first NMOS transistor is configured to connect to a drain terminalof a current mirror NMOS transistor that is part of a current mirrorconfiguration.
 16. The memory system of claim 14, wherein the PMOStransistors of each of the plurality of input level detection circuitbranches have the same size, and wherein the NMOS transistors of each ofthe plurality of input level detection circuit branches have the samesize.
 17. The memory system of claim 13, wherein the variable frequencygenerator includes a tunable ring oscillator circuit.
 18. A method,comprising: comparing an output voltage of a charge pump device to aplurality of predefined input detection voltage levels; based on thecomparison, causing one or more of a plurality of enable signals to turnon; based on the enable signals that are turned on, selecting a clocksignal frequency from a plurality of predetermined frequencies;outputting the selected clock frequency to the charge pump device; andmodifying the output voltage of the charge pump device based on theselected clock frequency.
 19. The method of claim 18, wherein the clocksignal frequency is selected using a tunable ring oscillator circuit.20. The method of claim 18, wherein selecting a clock signal frequencythat is higher in frequency than the current frequency increases thedrivability of the charge pump device and wherein selecting a clocksignal frequency that is lower in frequency than the current frequencydecreases the drivability of the charge pump device.